Known rules to reduce stress of fish moving in passageways are as follows:
1) Fish should not be confined in densities greater than 3 lbs of fish per cubic foot of water—to minimize their rubbing together;
2) Air should be available at the surface where fish are held;
3) Fish should enter a vessel from below water level;
4) Fish should not experience abrupt changes in direction—radius of turns should be at least 5 times conduit diameter;
5) Fish should not be vacuumed nor pumped; and
6) Rest time should be adjustable for fish species and changing conditions.
Known desirable features less commonly found, possibly because of associated high cost with prior art passageways, include:
1) Water temperature control at the fish outlet to acclimatize fish to their journey downstream;
2) Fish control with variable control of light, sound, and pulsed direct electric current at the boundaries of a passageway channel;
3) Shielding from solar heating; and
4) Fish counters.
Fish passage planning software is available on the United States federal fisheries website for culverts in series, at gradients of about 1 percent, along with data on a fish length and swim speeds for a wide range of fish species.